In recent years, since an organic thin film solar cell which is made of an organic photoelectric conversion element can be formed by a coating method, it has been attracted attention as a solar cell suitable for mass production, and intensive investigation has been made in many research institutions. The organic thin film solar cell was improved in a charge separation efficiency which had been a problem by adopting what is called a bulk heterojunction structure in which an electronic donor material and an electronic acceptor material were mixed (for example, refer to Patent document 1). In recent years, the photoelectric conversion efficiency was improved to achieve to an extent of 5 to 6%, and it can be said that the investigations towards a practical application has been activated more. However, in the organic photoelectric conversion element towards a future practical application, development of the organic photoelectric conversion element which generates electricity at higher efficiency is desired.
Moreover, when manufacturing an organic thin film solar cell with a coating process using a flexible substrate, it is possible to manufacture an inexpensive solar cell with a low manufacturing cost which cannot be realized with the conventional silicone system solar cell by applying coating process so called a roll to roll production process. However, for the purpose of practical realization, development of an inexpensive and highly efficient flexible transparent electrode substrate is one of the key investigational works.
Conventionally, as a transparent electrode, there has been mainly used an ITO transparent electrode having an indium-tin complex oxide (ITO) membrane produced by a vacuum deposition method or a sputtering process on transparent base materials, such as glass and a transparent plastic film. However, it was problems that the transparent electrode formed using a vacuum deposition method or sputtering process has inferior manufacturing efficiency, and the manufacturing cost is high and that it is inapplicable to the device application in which a flexible property is required since it is inferior to flexibility.
In order to solve this problem, it was proposed a method to form a transparent electrode by coating metal oxide particles such as ITO (for example, refer to Patent document 2.). However, it remained a problem that sufficient conductivity for functioning as an electrode could not be acquired. Although it is possible to lower electrical resistance by carrying out baking treatment of the coated film of metal oxide particles, this treatment cannot be applied, when a transparent electrode is formed on a resin base material like a plastic film.
Moreover, as a method of forming a transparent electrode excellent in manufacturing efficiency, there was proposed a method for forming a transparent electrode with a wet coating method or with a printing method using a coating liquid which dissolved or dispersed a conductive polymer represented by pi conjugated polymer in a suitable solvent as the production way of a transparent electrode (for example, refer to Patent document 3.). However, the transparent electrode obtained by the method proposed in Patent document 3 had a problem that conductivity was lower and also clarity was inferior compared with a metal oxide transparent electrode such as ITO formed by a vacuum film formation method.
There is disclosed a technique using an electro conductive fiber like a carbon nanotube (CNT) or a metal nanowire. Here, fixing a part of an electro conductive fiber is fixed to a substrate with a transparent resin layer, and a part of the electro conductive fiber is projected on the surface of the transparent resin layer to result in forming an electrode (for example, refer to Patent documents 4 and 5). However, since the electrode of such composition has electro conductivity only in the part from which the electro conductive fiber is projected on the surface, it does not have a function as a flat electrode. In addition, since the electro conductive fiber is projected on the surface and too much electrical resistance will be loaded on the projected part, the deterioration from a projected part will occur and this method had the problem that the durability of the whole element was inferior.